Date: Thu, 21 Jun 2001 08:40:45 -0500

Author: trappe@physics.utexas.edu (Karl Trappe)

Subject: Re: unknown demo

Post:

The force on the bar is F=ilXB, where i=current in the bar, l=length of
bar, B=magnetic field of other bar. B supplied by other bar is
B=mu*i/2*pi*r, where i= current in stationary bar, and r is the separtion
between the bars at the instant in question. Clearly, since r changes,
this becomes an integral from r1 to r2, if you want to get the total forces
moving the bar while the current is connected.

Symmetry allows you to exchange which bar is which, but you've got one
fixed, and the other moveable, so that is the usual explanation. Unless,
of course, I'm still asleep...Karl

>Hi all,
>
>Could someone please enlighten me as to the name and details of a demo
>that I have?
>
>A current is supposed to pass through parallel rods, upon which are sitting a
>movable (rolling) bar. The movable bar is sitting in a permanent magnetic
>field. When a small voltage is applied, a current flows, and the bar
>rolls back
>toward the supply.
>
>I understand the "basic" concept of interacting fields between the parallel
>conductors and conductors trying to force themselves apart because of this
>interaction. But, why does the interaction of the magnetic field cause
>the rod
>to roll in one direction? Is this demonstrating Lenz's law?
>
>Thanks for any replies, I hope the question is not to dense!
>
>Regards
>Liane
>Physics Technician
>Faculty of Health & Sciences
>CPIT
>williamsl@cpit.ac.nz
>3649000 extn 8330

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